1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle having an engine for generating propulsive forces for the hybrid vehicle and an electric motor for generating assistive forces in addition to the propulsive forces generated by the engine and absorbing the propulsive forces generated by the engine.
2. Description of the Related Art
There has in recent years been developed a hybrid vehicle which has an engine, like an internal combustion engine for ordinary automobiles, as a power source for generating propulsive forces (running drive forces) for the hybrid vehicle and also an electric motor connected to the output shaft of the engine for generating assistive drive forces in addition to the propulsive forces generated by the engine, i.e., applying assistive drive forces to the output shaft of the engine, and for being operated as an electric generator by the engine (regenerative mode) to convert and absorb the propulsive forces generated by the engine as electric energy. To generate the assistive drive forces, the electric motor is energized by electric energy from an energy storage unit such as a storage battery. When it is operated as the electric generator, the generated electric energy is stored in the storage battery.
One such hybrid vehicle is disclosed in Japanese patent publication No. 6-1925, for example. In the disclosed hybrid vehicle, when the driver sets up a motor mode by operating a given switch, the electric motor is controlled to generate assistive drive forces depending on the depth to which the driver presses the accelerator pedal. When the driver sets up a regenerative mode by operating a given switch, the electric motor is controlled to generate electric energy for applying braking forces depending on the position of the switch, and part of the generated electric energy is stored in the storage battery.
Since the assistive drive forces produced by the electric motor are determined simply by the depth to which the driver presses the accelerator pedal, when the depth to which the driver presses the accelerator pedal is abruptly changed, the assistive drive forces produced by the electric motor are also abruptly changed. Therefore, the behavior of the hybrid vehicle as it runs tends to be sluggish. Furthermore, since a discharged current of the storage battery is also abruptly changed when the assistive drive forces produced by the electric motor are abruptly changed, the storage battery is apt to be short in service life.
In the motor mode, the electric motor does not generate electric energy. Therefore, the stored energy of the storage battery tends to be reduced, with the result that the storage battery will fail to supply enough energy to the electric motor, which will then fail to produce sufficient assistive drive forces. Inasmuch as the electric motor produces assistive drive forces only when the driver sets up the motor mode with the switch, if the driver forgets to operate the switch or keeps the regenerative mode turned on, then the electric motor cannot produce desired assistive drive forces.
In the regenerative mode, the braking forces produced by the generation of electric energy with the electric motor are determined by the position to which the switch is shifted by the driver. Therefore, if the driver does operate the switch properly, then the electric motor may produce smaller braking forces than would be necessary or larger braking forces than would be necessary.
Because the amount of generated electric energy which governs the braking forces is determined by the position to which the switch is shifted by the driver, the electric motor may possibly generate too a small amount of electric energy while the stored energy of the storage battery is small, and the stored energy of the storage battery may remain insufficient. Conversely, the electric motor may possibly generate too a large amount of electric energy while the stored energy of the storage battery is large, and the excessive electric energy may be wasted.